Marten M., Wijnant Y., de Boer A.

Previous research of the authors pointed out that side-resonators can be applied to reduce fan noise. However, the noise reduction capabilities of most resonator geometries, e.g. tube resonators, cylindrical resonators (cylindrical air layers) and circular resonators (disc shaped air layers), are relatively narrow banded. This is disadvantageous in case resonators are used in combination with a noise source that emits broadband noise or tonal noise at varying frequencies (for instance a speed controlled fan). It was found that the choice of the resonator geometry influences the broadband reduction capabilities (circular resonators offering the best broadband reduction capabilities). In the present study it is investigated to what extend elliptical resonators, consisting of an elliptical shaped air layer, can be used to achieve broad noise reduction. A semi-analytical model is proposed that describes the wave propagation in the elliptical shaped air layer. This model is connected to the analytical solution for wave propagation in a tube. The dimensions of the elliptical resonator can be optimized for broadband noise reduction with this model. In addition an experimental setup and a finite element model of the elliptical resonator including viscous elements are used to verify the semi-analytical model of the elliptical resonator.